Cystic Fibrosis (CF) is a fatal disease caused by a defective gene that encodes for CF transmembrane conductance regulator (CFTR), a glycoprotein important in chloride transport. Approximately 75% of CF patients carry a deletion of phenylalanine from position 508 of the coding sequence for CFTR. This mutant CFTR is incompletely processed within the endoplasmic reticulum with limited transport of nonfunctinal CFTR to the airway epithelial surface, thereby causing defective chloride secretion. "Protein assist" therapy, such as CPX is a new approach to activate chloride secretion via the mutant protein channels. CPX binds with high specificity and affinity to the first nucleotide binding domain of CFTR and restores function to mutant CFTR molecules in vitro. This Phase II study will evaluate the safety and efficacy of multiple ascending oral doses of oral CPX in adults with CF.